The present invention relates to a thermally processed image forming material, and more particularly to the material suitable for photographic printing plate making using a scanner or image setter, or for medical purposes, and still more particularly to the material for photographic printing plate making or medical purposes having an excellent coated surface property and being capable of producing images with a low fog and high Dmax (maximum density).
There are a variety of known photosensitive materials having on a support a photosensitive layer and producing an image by image exposure. Of these, a technique for producing image by heat development is worth a particular mention in that allowing environmental preservation and simplifying the image producing means.
A strong need for reducing the volume of waste process solution has arisen in recent fields of photographic printing plate making or medical diagnosis from viewpoints of environmental preservation and space saving. Thus a technology related to a photothermographic material for photographic printing plate making or medical diagnosis has been desired, the material being such that affording efficient laser exposure and providing a clear black image with high resolution and sharpness. Such photothermographic material can provide the user with a more simple and environment-conscious heat development and processing system using no solution-base process chemical.
The image producing method based on heat development is disclosed, for example, in U.S. Pat. Nos. 3,152,904 and 3,457,075 and xe2x80x9cThermally Processed Silver Systemsxe2x80x9d written by D. Morgan and B. Shely, Imaging Processes and Materials, Neblette""s 8th ed., edited by Sturge, V. Walworth and A. Shepp, Chapter 9, p.279, (1989). Such photosensitive material contains an reducible non-photosensitive silver source (e.g., organic acid silver salt), a catalytic amount of photocatalyst (e.g., silver halide) and a reducing agent for silver, all of which being generally dispersed in an organic binder matrix. While the photosensitive material is stable at the room temperature, it will produce silver through a redox reaction between the reducible silver source (which serves as an oxidizing agent) and the reducing agent when heated to a high temperature (80xc2x0 C. or above, for example) after light exposure. The redox reaction is promoted by a catalytic action of the latent image produced by the light exposure. That is, the silver generated by the reaction of the reducible silver within the exposed area provides a black spot, which makes a contrast with the non-exposed area and is recognizable as an image.
The silver source employed by such system generally refers to a silver salt of a fatty acid, and a variety of methods for producing thereof have been known. Examples of the methods include such that preparing an organic acid silver salt in a concomitant solution of water and water-insoluble solvent as disclosed for example in JP-A-49-93310 (the code xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d), JP-A-49-94619 and JP-A-53-68702; such that preparing an organic acid silver salt in an aqueous solution as disclosed in JP-A-53-31611, JP-A-54-4117 and JP-A-54-46709; and such that preparing an organic acid silver salt in an organic solvent as disclosed in JP-A-57-186745, JP-A-47-9432 and U.S. Pat. No. 3,700,458. In principle, the organic acid silver salt is obtained by dissolving a fatty acid into water under heating to a temperature of the melting point thereof or above, adding sodium hydroxide or an alkali metal salt under vigorous stirring, and further adding silver nitrate to convert an alkali soap into a silver soap.
The alkali soap forms micell in the aqueous solution, which appears as a milky liquid. The conversion reaction from such micellar state to silver salt, however, often suffers from a problem in production stability. Thus as a measure for obtaining a homogeneous solution of alkali soap, use of a mixed solution of water and alcohol as a solvent is disclosed in JP-A-55-40607.
Now the alkali soap shows alkalinity as its name suggests, so that the silver soap is prepared under a high pH environment. Adding silver nitrate to an alkali solution, however, not only produces silver oxide as a by-product but also results in an undesirable production of silver nucleus by an action of a trace amount of contaminant which inevitably generates during the production and exhibits a high reducing activity under such high-pH environment. Such by-product is quite disadvantageous in that degrading property of the thermally processed photographic material, and more specifically in that causing undesirable fog and degrading the coated surface quality. From this viewpoint, a method for obtaining a homogeneous solution to suppress the generation of the by-product is disclosed in JP-A-55-40607, in which fog still remains unsolved.
In JP-A-9-127643, disclosed is a method for producing silver salt based on simultaneous measuring and addition of an alkali metal salt solution and silver nitrate solution, and is specified as simultaneous addition of an aqueous sodium behenate solution and isopropyl alcohol. While the method is successful in at least lowering the high pH during the reaction to the medium range and thereby in suppressing the generation amount of salver oxide, fog still cannot thoroughly be cleared and the coated surface quality still cannot be improved due to a weak reducibility of isopropyl alcohol.
As described above, preparation of fatty acid silver salt needs special accounts such that eliminating as possible reducible substances during the formation of fatty acid silver salt, controlling the grain size and controlling the grain form, where all these requirements cannot be satisfied at a time by the conventional method.
In the conventional production of a thermally processed material using the fatty acid silver salt, a photosensitive layer thereof is often formed by coating a coating liquid containing an organic solvent such as toluene, methyl ethyl ketone or methanol. Using an organic solvent as the solvent, however, is not only disadvantageous in terms of safety in the production processes, adverse effects on human body, and high cost ascribable to the solvent recovery or the like, but is also inappropriate in terms of providing an environment-conscious photothermographic material.
Thus a method for forming, using a water-base coating liquid, the photosensitive layer (also referred as xe2x80x9cwater-base photosensitive layerxe2x80x9dhereinafter) is proposed. For example, JP-A-49-52626 and JP-A-53-116144 disclose cases using gelatin as a binder. In JP-A-50-151138, a case using polyvinyl alcohol as a binder is described.
A case with a combined use of gelatin and polyvinyl alcohol is found in JP-A-60-61747. As another exemplary case, the photosensitive layer using a water-soluble polyvinyl acetal as a binder is described in JP-A-58-28737.
Using a water-soluble binder allows the photosensitive layer to be formed with a water-base coating liquid and is beneficial from environmental and economic viewpoints. The water-soluble polymer binder is, however, less compatible with the fatty acid silver salts, and may interact with the photographic additives other than the organic silver salt, which are usually dissolved or dispersed also in a water-base solvent, to produce an undesirable agglomeration and thereby to make it difficult to obtain a good surface quality.
In order to obtain practically agreeable quality of the coated surface using the water-base coating liquid containing a fatty acid silver salt, the fatty acid silver salt must be kept in a finely dispersed state in the water-base solution without causing agglomeration. Discovery of a method for finely dispersing the fatty acid silver salt is thus desired. One method generally accepted relates to such that producing a hydrophobic grain dispersion of a fatty acid silver salt, separating the grain therefrom by filtration to obtain a solid matter, and re-dispersing the solid matter after being mixed with a dispersing agent as described by D. Kloosterboer in Imaging Processes and materials, Neblette""s 8th ed., edited by Sturge, V. Walworth and A. Shepp, p.279, (1989).
Fine dispersion operation of the fatty acid silver salt can be effected by mechanical dispersion in the presence of a dispersing agent using a known pulverizing means (e.g., high-speed mixer, homogenizer, high-speed impact mill, banbury mixer, homomixer, kneader, ball mill, vibration ball mill, epicyclic ball mill, attritor, sand mill, bead mill, colloid mill, jet mill, roller mill, trommel and high-speed stone mill). These methods, however, produce only a coating liquid including a lot of agglomerated grains and are thus causative of degraded surface quality, and, worse than all, tend to indiscriminately cleave the primary grains of the fatty acid silver salt which are originally crystallized as a water-insoluble salt, so that excessive silver nuclei are generated on the crystal cleavage plane of the grains and thereby to increase fog.
On the other hand, JP-B-7-119953 (the code xe2x80x9cJP-Bxe2x80x9d as used herein means an xe2x80x9cexamined Japanese Patent Publicationxe2x80x9d), JP-A-8-137044 and JP-A-8-238848 disclose methods such that finely dispersing the fatty acid silver salt by pressure treatment. The methods, however, relate to an organic solvent-base dispersion and stand on a concept different from solving the foregoing problem.
In JP-A-9-127643, disclosed is a method such that obtaining a dispersion of the fatty acid silver salt by simultaneous measuring and addition of an alkali metal salt solution and silver nitrate solution, and then directly desalting the dispersion by dialysis or ultra-filtration. This method is preferable at least in that the primary grain obtained in the crystallization process of the fatty acid silver salt can be incorporated as intact into the photosensitive layer without being crushed. The method, however, still suffers from problems in agglomeration of the grains under a condition of high salt concentration, and in thickening during concentration of the dispersion, which makes the method difficult to be accepted as a measure for obtaining a practical coating liquid.
Another problem resides in that vigorous stirring is required when the alkali metal salt solution and silver nitrate solution are mixed in order to obtain a fine and monodisperse grains of the fatty acid silver salt. In particular, since a solution of a fatty acid alkali metal salt dissolved at a high temperature will instantaneously deposit crystal due to abrupt cooling upon the addition, a slow dilution speed and moderate fluidization will undesirably result in large and coarse grains. Raising the stirring speed during the addition into a tank in which a gas/liquid interphase is formed, however, causes entrainment of the air. Since the fatty acid silver salt grains are strongly hydrophobic and will adhere on the surface of the entrained air bubbles, which not only prevents bubble rupture but also causes agglomeration of adjacent grains on the surface of the bubbles. The liquid such entraining the air appears like a whipped cream, and for the case of desalting the by-produced salt through ultra-filtration, this will significantly degrade the handling property, and the agglomerated grains will clog the filtration membrane.
Temperature of the reaction liquid after the reaction between the silver ion-containing solution and the solution of a fatty acid alkali metal salt is preferably kept around the room temperature, since too high temperature will result in growth of the grains by a physical ripening process. Whereas, the temperature needs be kept at 50xc2x0 C. or above to obtain a stable solution of an alkali metal salt of a long-chained fatty acid, so that it is necessary to ensure a rapid heat exchange so as to cancel an incoming heat introduced with the added liquid. In this point, a measure for providing a jacket vessel to a tank or the like suffers from a problem in that a heat-exchangeable area educes as volume of the reaction liquid increases, which makes it difficult to scale up the production process.
As described above, a stable method for preparing a water-base coating liquid containing fatty acid silver salt grains capable of affording the thermally processed image forming material with an excellent coated surface quality and optical properties such as low haze and low fog has not been discovered yet.
It is therefore an object of the present invention to provide a thermally processed image forming material allowing fabrication by water-base coating, which is environmentally and economically advantageous, having an excellent coated surface property, and being capable of producing an image with low fog and high black density.
The present inventors found after extensive investigations for achieving the above object that an excellent thermally processed image forming material capable of affording a desired effect can be obtained by incorporating non-photosensitive fatty acid organic silver salt grains; where (1) the grains being obtained by mixing and reacting a silver ion-containing aqueous solution with a solution of a fatty acid alkali metal salt within a closed liquid mixing means or (2) by micro-dispersing the reaction mixture at a predetermined operating pressure using a ultrahigh pressure dispersion apparatus; such findings led us to propose the present invention.
That is, the present invention is to provide a thermally processed image forming material containing elsewhere on a support a reducing agent, a binder and non-photosensitive fatty silver salt grains characterized in that (1) the non-photosensitive fatty acid silver salt grains are prepared by mixing and reacting a silver ion-containing solution, the solvent of which being water or a mixture of water and an organic solvent, with a solution of a fatty acid alkali metal salt, the solvent of which being water, an organic solvent, or a mixture of water and an organic solvent, in a closed mixing means, or (2) the non-photosensitive fatty acid silver salt grains are prepared by micro-dispersing the reaction mixture at an operating pressure of 1,800 kg/cm2 or above using a ultrahigh pressure dispersion apparatus.
In a preferred embodiment of the present invention, the non-photosensitive fatty silver salt grains are prepared by cooling a reaction mixture obtained after the reaction proceeded within the closed mixing means. In another preferred embodiment of the present invention, the non-photosensitive fatty acid silver salt grains are prepared by micro-dispersing the reaction mixture, obtained after the reaction proceeded within the closed mixing means, at an operating pressure of 1,800 kg/cm2 or above using a ultrahigh pressure dispersion apparatus, and in particular, in a ultrahigh pressure jet flow.
The thermally processed image forming material of the present invention preferably contains an additional silver halide.
The thermally processed image forming material of the present invention preferably has an image producing layer which contains the non-photosensitive fatty silver salt grains and the binder, and a polymer latex having a glass transition point of xe2x88x9230xc2x0 C. to 40xc2x0 C. comprises 50 wt % or more of the binder. Note in this specification that a certain range of values expressed using a word xe2x80x9ctoxe2x80x9d always includes both end values given before and after thereof.
The thermally processed image forming material of the present invention preferably contains at least a single species of nucleation aid in at least one layer provided on the same side of the image producing layer on the support.
The nucleation aid is preferably at least any one of a compound selected from the group consisting of a substituted alkene derivative expressed by the general formula (1) below, a substituted isooxazole derivative expressed by the general formula (2) below, and an acetal derivative expressed by the general formula (3) below: 
(where, R1, R2 and R3 independently represent a hydrogen atom or substituent: Z represents an electron attracting group or silyl group; and, R1 and Z, R2 and R3, R1 and R2, or R3 and Z may individually bind with each other to form a cyclic structure), 
(where, R4 represents a substituent), and 
(where, X and Y independently represent a hydrogen atom or substituent; A and B independently represent alkoxy group, alkylthio group, alkylamino group, aryloxy group, arylthio group, anilino group, heterocyclic oxy group, heterocyclic thio group or heterocyclic amino group; and, X and Y, or A and B may individually bind with each other to form a cyclic structure).